Non-contact optical system, computer-accessible medium and method for measurement at least one mechanical property of tissue using coherent speckle technique(s)

ABSTRACT

Exemplary embodiments of apparatus and method for determining at least one material property of an anatomical structure can be provided. According to one exemplary embodiment, it is possible to apply at least one first coherent radiation to at least one portion of the anatomical structure, and receive at least one second coherent radiation from such portion(s). The first and second coherent radiations can be associated with one another. In addition, it is possible to determine the material property based on the second coherent radiation(s). Such determination can be performed without (i) any portion of an apparatus performing the procedure causing an induction of at least one mechanical deformation on or in the anatomical structure, and/or (ii) any mechanical deformation on or in the anatomical structure.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This present application relates to and claims priority from International Patent Application No. PCT/US2010/027193 filed Mar. 12, 2010, and from U.S. Patent Provisional Application Ser. No. 61/159,474, filed on Mar. 12, 2009, the entire disclosures of which are hereby incorporated herein by reference.

FIELD OF THE DISCLOSURE

Exemplary embodiments of the present disclosure relates generally to measuring properties associated with tissues, and more particularly to non-contact optical system, computer-accessible medium and method for measuring at least one mechanical or material property of tissue using laser speckle.

BACKGROUND INFORMATION

In many pathological disease processes, the material properties of tissue are altered from a normal state. The development of techniques to measure the mechanical or material properties of tissue can potentially facilitate disease diagnosis and guidance of therapy. The system and methods described in this embodiment can potentially be applied for diagnosis of a variety of disease processes. To describe the technique, in this embodiment it is possible to focus on the cardiovascular applications for the detection of unstable atherosclerotic plaque.

Atherosclerotic Plaque Rupture and the Role of Biomechanical Factors: Despite widespread efforts towards its detection and therapy, thrombus mediated ischemic cardiovascular disease still remains the leading cause of mortality in industrialized societies. The rupture of unstable coronary atherosclerotic plaque frequently can precede a majority of ischemic cardiovascular events. The mechanisms leading to plaque rupture can be multi-factorial involving a complex liaison between morphological, compositional, biochemical and biomechanical processes. Due to the cumulative effect of multiple factors, the mechanical stability of the plaque is compromised resulting in an elevated risk of rupture. It is believed that during atherosclerotic plaque progression, the intrinsic mechanical properties of the plaque are serially altered and the measurement of a metric to accurately evaluate intrinsic plaque mechanical properties provides a key determinant of plaque stability. This belief can be based on evidence that mechanical factors greatly influence plaque stability. Hemodynamic forces affect wall shear stresses influencing plaque progression, susceptibility to plaque rupture and coronary thrombosis.⁶ Finite element studies have suggested that rupture of the fibrous cap is greatly influenced by regions of high circumferential stress typically in the lateral cap shoulders. The morphology and mechanical properties of the atheroma can affect stress distributions, with plaque rupture frequently occurring in focal regions of high stress concentrations caused by large differences in intrinsic mechanical properties of the fibrous cap and lipid pool. The mechanical properties of the atheroma determine the extent of induced deformation or strain in response to an extrinsic load. Higher strains are measured in lipid rich regions of lower viscosity. Cyclic mechanical strain within the arterial wall affects macrophage gene expression and SMC proliferation. Histology studies have shown the localization of MMP-1 in regions of high circumferential strain within plaques, suggesting that mechanical properties influence MMP release further weakening plaque structure contributing to a greater tendency towards plaque rupture.

Image-based methods to measure arterial mechanical properties: A variety of techniques such as intravascular ultrasound (IVUS), virtual histology (VH)-IVUS, magnetic resonance imaging (MRI), angioscopy, thermography, near infrared (NIR) and Raman spectroscopy have been investigated for evaluating coronary plaques in patients. High resolution optical techniques such as optical coherence tomography (OCT) and its next generation implementation, optical frequency domain imaging (OFDI) can provide the opportunity to evaluate plaque microstructure and identify TCFA's in patients. These technologies provide invaluable information on microstructural, compositional and inflammatory factors related to plaque instability and are complementary to approaches that measure mechanical factors. To address the specific need for evaluating mechanical factors, IVUS elastography and finite element analysis (FEA) techniques have been developed. IVUS elastography computes local strains in the arterial wall in response to intra-luminal pressure differentials using cross-correlation analysis and estimation of tissue velocity gradients. Elastography approaches have been applied to OCT to potentially provide higher spatial resolution of strain estimation relative to IVUS. FEA approaches can utilize computer-generated models based on OCT or IVUS cross-sections and estimates of tissue material properties for modeling intra-plaque stress/strain distributions. These techniques provide important information in that they enable the measurement of plaque response to a dynamic external loading environment, thus aiding the investigation of plaque instability. However, the measurement of plaque viscoelasticity using these approaches is intractable, requiring a priori guesstimates of viscoelastic properties, and knowledge of microstructure and loading conditions to solve the inverse problem.

Viscoelasticity and Brownian Motion: Tissue is viscoelastic in nature, exhibiting both solid and fluid like characteristics. The mechanical properties of viscoelastic materials can be evaluated by measuring a quantity, termed the “viscoelastic modulus”, which determines the strain induced in the material in response to an extrinsic load. Traditionally, the viscoelastic modulus is measured using a mechanical rheometer, in which a material is loaded between two parallel plates, an oscillatory stress at frequency, ω, is applied and the a strain response is measured to evaluate viscoelasticity. The measured viscoelastic modulus, G*(ω), is expressed as, G*(ω)=G′(ω)+iG″(ω). The real part, G′(ω), is the elastic modulus which defines the elastic solid like characteristics of the material and is the ratio of the elastic component of the oscillatory stress which is in phase with the strain. The imaginary part, G″(ω), provides the viscous modulus and measures the out-of-phase response of the medium defining the material's fluid like characteristics. The ratio between the elastic to viscous moduli provides a measure of ‘phase’, where a lower phase represents a more elastically dominated and a higher phase represents a more viscously dominated material.

Studies in the field of polymer rheology have demonstrated non-contact approaches to measure the viscoelastic modulus by evaluating the passive movements (Brownian motion) of particles suspended in a viscoelastic medium. In one publication, it was demonstrated that the Brownian motion of suspended particles is intimately related to the structure and viscoelastic properties of the suspending medium, and particles exhibit larger range of motions when their local environment is less rigid. This indicated that the response of a viscoelastic material to the average Brownian motion of dispersed microscopic particles closely resembles the response of the material to an imposed oscillatory mechanical load at frequency, ω. Consequently, other studies have indicated the use of light scattering techniques to evaluate the viscoelastic modulus of homogenous polymer materials by suspending exogenous particles and measuring the time scale and mean square displacement of microscopic trajectories. By applying these concepts, a further exemplary optical technique can be reviewed, e.g., termed Laser Speckle Imaging, which analyzes the intrinsic Brownian motion of endogenous microscopic light scattering particles that are inherently present within tissue to evaluate tissue viscoelasticity.

Laser Speckle Imaging (LSI): When an object is imaged using highly coherent light from a laser, a granular pattern of multiple bright and dark spots becomes apparent on the image, which bears no perceptible relationship to the macroscopic structure of the object. These random intensity patterns, termed as laser speckle, can occur in two situations: (i) when coherent light is reflected from a surface which is rough on the scale of an optical wavelength, and (ii) when coherent light propagates through and is scattered by a medium with random refractive index fluctuations such as in tissue. The interference of light returning from the random surface or medium causes laser speckle. Laser speckle formed from scattering within tissue is exquisitely sensitive to Brownian motion. The Brownian motion of endogenous light scattering particles in tissue causes scatterer locations and optical path lengths to dynamically change resulting in time dependent intensity modulations of laser speckle. The rate of laser speckle modulation can be highly dependent on the extent of motion of suspended scatterers, which is in turn influenced by viscoelasticity of the medium. Consequently, in an atheroma, due to the relatively low viscosity of lipid, endogenous scatterers within the compliant necrotic core exhibit more rapid Brownian motion compared to the stiffer fibrous regions of the plaque. Since scatterer motion governs the modulation of laser speckle, the measurement of temporal intensity variations of laser speckle patterns provides information about the viscoelastic properties of the plaque. Using these principles, the measurement of intensity modulations of time-varying laser speckle patterns can provide a highly sensitive technique for evaluating atherosclerotic plaques. Exemplary procedures using excised atherosclerotic plaques have been reviewed, indicating that the measurement of intrinsic Brownian motion of endogenous particles, related to viscoelasticity, can be used to distinguish plaque type, and evaluate collagen and lipid content.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE DISCLOSURE

For example, (a) exemplary embodiments of the LSI techniques and systems according to the present disclosure can be provided for plaque characterization and identification of high-risk plaque, (b) the exemplary LSI time constant can be related to collagen and lipid content, (c) exemplary embodiments of the LSI techniques and systems according to the present disclosure can measure an index of viscoelasticity that can be related to the viscoelastic modulus, G*, (d) fibrous cap thickness can be measured using LSI (e) exemplary embodiments of the LSI techniques and systems according to the present disclosure can identify high-risk plaques during physiological arterial deformation, and (f) the apoE knockout mouse can provide a useful model to evaluate plaque progression. It is likely that exemplary embodiments of LSI techniques and system can provide an exemplary platform for measuring composite metrics of plaque stability based on biomechanical, structural and compositional factors. Exemplary measurements of time constant can be performed by fitting a single exponential to a portion of the normalized speckle decorrelation curve.

For example, by evaluating contributions of different time constants using multiexponential analysis of speckle decorrelation, it is possible to increase the efficacy of LSI in investigating plaque heterogeneity. The use of spatio-temporal analysis of exemplary embodiments of the LSI techniques and systems according to the present disclosure in providing depth information has been shown. When combined with beam scanning, this exemplary feature of the exemplary embodiments of the LSI techniques and systems can facilitate the measurement of three-dimensional maps of plaque viscoelasticity and morphology. Studies have shown that the measurement of the mean square displacement of particle trajectories obtained from diffuse light scattering techniques can be used to calculate the viscoelastic modulus in homogenous polymer solutions. These exemplary principles can be applied to determine the intrinsic viscoelastic modulus of tissue components from laser speckle patterns.

One of the objects of certain exemplary embodiments of the present disclosure is to provide quantitative indices based on plaque biomechanical properties using LSI to determine the risk of plaque rupture. While certain preliminary studies have successfully demonstrated the capability of LSI in diagnosing plaque type, in order to realize the exemplary objects of the present disclosure, the exemplary embodiment of the LSI techniques and systems can facilitate its use to accurately quantify plaque viscoelasticity. According to one exemplary embodiment of the present disclosure, it is possible to facilitate exemplary methods and systems for measuring the viscoelastic properties of arterial tissue from, e.g., laser speckle images and compare our results with standard mechanical testing measurements.

According to another exemplary embodiment of the present disclosure, it is possible to determine changes in plaque viscoelasticity using the exemplary embodiments of the LSI systems and method during different stages of arterial atherosclerotic plaque progression in an atherosclerotic mouse model. Some of the capabilities of LSI that facilitate the measurement of tissue mechanical properties are listed: ♦Exemplary LSI techniques and system can measure intrinsic Brownian motions of endogenous scatterers providing measurements that are intimately linked with the micro-mechanical behavior of the tissue. ♦Exemplary LSI techniques and systems can be implemented using a relatively inexpensive laser source and a high-speed CMOS or CCD camera, enabling the study of tissue viscoelastic behavior ‘in situ’ over a large frequency range over several kHz, defined by the frame rate of the detector. ♦Exemplary LSI measurements can be sensitive to small changes in the viscoelastic properties of the tissue because speckle decorrelation induced by phase shifts in highly scattering media requires very minute displacements of scatterers at length scales smaller than the optical wavelength. ♦Beam scanning enables the unique ability to measure 2D distributions of tissue viscoelastic behavior. ♦High-speed CMOS or CCD technology (˜kHz acquisition) enables speckle decorrelation measurements over very short time scales (few ms) over which the influence of low frequency arterial deformations induced by cardiac (˜1 Hz) or respiratory (˜0.2 Hz) motion is largely mitigated. ♦Exemplary LSI techniques and systems can be utilized implementing small-diameter optical fiber bundles, thus elegantly lending itself for intracoronary applications.

It is also possible to apply the exemplary techniques described herein to other optical methods that utilize coherent sources including optical methods such as, e.g., OCT, OFDI, SD-OCT and FD-OCT. The techniques described herein can also be applied to other methods that utilize other coherent radiation sources such as acoustic radiation including ultrasound. Ultrasound speckle patterns can be similarly analyzed to evaluate the viscoelastic properties of tissues by measuring ultrasound speckle decorrelation over finite time durations.

Exemplary embodiments of the LSI techniques and systems according to the present disclosure can be accurate for the detection of thin-cap fibroatheromas, and for measuring necrotic core area, fibrous cap thickness and plaque morphology ex vivo.

Exemplary LSI techniques and systems according to the present disclosure can be accurate for the detection of thin-cap fibroatheromas, and for measuring necrotic core area, fibrous cap thickness and plaque morphology ex vivo.

According to one exemplary embodiment of the present disclosure, it is possible to use such knowledge to measure the viscoelastic modulus, provide technology to conduct intra-arterial LSI in vivo, and derive key biomechanical markers for early detection of high-risk plaques.

Indeed, exemplary embodiments of apparatus and method for determining at least one material property of an anatomical structure can be provided. According to one exemplary embodiment, (e.g., using at least one first arrangement) it is possible to apply at least one first coherent radiation to at least one portion of the anatomical structure, and receive at least one second coherent radiation from such portion(s). The first and second coherent radiations can be associated with one another. In addition, (e.g., using at least one second arrangement) it is possible to determine the material property based on the second coherent radiation(s). Such determination can be performed without (i) any portion of an apparatus performing the procedure causing an induction of at least one mechanical deformation on or in the anatomical structure, and/or (ii) any mechanical deformation on or in the anatomical structure.

According to one exemplary embodiment of the present disclosure, the first and/or second coherent radiation(s) can be an electro-magnetic radiation. It is possible to scan the anatomical structure at multiple locations, e.g., simultaneously and/or sequentially. It is also possible to detect a scan of the anatomical structure at the multiple locations simultaneously and/or sequentially.

In another exemplary embodiment of the present disclosure, the material property can be spatially-varying or depth-varying, as well as an elastic property or a viscous property of the anatomical structure. Further, the material property can be a macroscopic property, a microscopic property and/or a mesoscopic property of the anatomical structure. Such material property can also be a strain on the anatomical structure.

According to still another exemplary embodiment of the present disclosure, it is possible (e.g., using the second arrangement) to determine the material property as a function of frequencies of motion of scatterers within the anatomical structure. The motion of the scatterers within the anatomical structure can be a Brownian motion.

In a further exemplary embodiment of the present disclosure, the first coherent radiation can be a multiply-scattered light, a single-scattered light, and/or coherent speckle. It is also possible (e.g., using the first arrangement) to apply the first coherent radiation(s) to at least one portion in-vivo. The first and/or second coherent radiation(s) can be an acoustic radiation.

These and other objects, features and advantages of the exemplary embodiment of the present disclosure will become apparent upon reading the following detailed description of the exemplary embodiments of the present disclosure, when taken in conjunction with the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present disclosure will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the present disclosure, in which:

FIG. 1 is an exemplary illustration of speckle patterns acquired from a thin-cap fibroatheroma (TCFA) showing time-dependent fluctuation of laser speckle;

FIG. 2(A) is an exemplary graph of Speckle decorrelation curves obtained for three exemplary aortic specimens: TCFA, thick-cap fibroatheroma (TKFA), and fibrous aortic plaques;

FIG. 2(B) is an exemplary chart illustrating mean τ computed for different plaque groups under static conditions;

FIG. 3 is an exemplary graph illustrating G* measured using a rheometer in response to a oscillatory load at frequencies less than 1 Hz;

FIG. 4(A) is an exemplary graph of a spatial heterogeneity in τ obtained by beam scanning over a necrotic core fibroatheroma;

FIG. 4(B) is an exemplary graph of the spatial heterogeneity in τ obtained by beam scanning over a calcific plaque;

FIG. 4(C) is an exemplary graph of the spatial heterogeneity in τ obtained by beam scanning over a fibrous plaque;

FIG. 4(D) is an exemplary map illustrating a distribution of speckle decorrelation time constants over a lesion compared with the accompanying gross pathology;

FIG. 5(A) is a graph of τ(ρ) is plotted vs. distance ρ from source;

FIG. 5(B) is an exemplary schematic illustration of a photon propagation through a two-layer model;

FIG. 6(A) is an exemplary block diagram of an exemplary embodiment of a method according to the present disclosure which can be used to measure and validate sample viscoelasticity using the exemplary LSI techniques;

FIG. 6(B) is an exemplary block diagram of an exemplary embodiment of a system according to the present disclosure which can be used to measure and validate the sample viscoelasticity using the exemplary LSI techniques;

FIG. 7(A) is an exemplary graph of frequency-dependent complex viscoelastic moduli measured from laser speckle patterns of fat, cartilage and skeletal muscle using the exemplary embodiments of the methods and systems according to the present disclosure;

FIG. 7(B) is an exemplary graph of frequency-dependent complex viscoelastic moduli measured from laser speckle patterns of calcific, fibrous and lipid-rich atherosclerotic plaques using the exemplary embodiments of the methods and systems according to the present disclosure;

FIG. 8(A) is an exemplary gross pathology photograph of a human aortic segment;

FIG. 8(B) is an exemplary map of the distribution of complex viscoclastic moduli measured at high frequency (˜250 Hz) by scanning a focused beam over the human aortic sample shown in FIG. 8(A);

FIG. 8(C) is an exemplary map of the distribution of complex viscoelastic moduli measured at frequencies ˜100 Hz by scanning a focused beam over the human aortic sample shown in FIG. 8(A);

FIG. 8(D) is an exemplary map of the distribution of complex viscoelastic moduli measured at lower frequencies ˜10 Hz by scanning a focused beam over the human aortic sample shown in FIG. 8(A); and

FIG. 9 is an exemplary graph of the spatial variation of speckle decorrelation time constant over a mouse aorta with a fibrous plaque.

Throughout the figures, the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the subject disclosure will now be described in detail with reference to the figures, it is done so in connection with the illustrative embodiments. It is intended that changes and modifications can be made to the described exemplary embodiments without departing from the true scope and spirit of the subject disclosure as defined by the appended claims.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary Design

Measuring viscoelasticity of atherosclerotic plaques: Characterization of Atherosclerotic Plaque using LSI: The exemplary capability of the exemplary embodiments of the LSI systems and methods according to the present disclosure for differentiating atherosclerotic plaque type, and assessing plaque morphology and composition is demonstrated has been described in, e.g., S. Nadkarni, et al., “Characterization of atherosclerotic plaques by laser speckle analysis”, Circulation, 2005. In this publication, 118 aortic specimens were obtained from 14 human cadavers using LSI. Light (632 nm) from a Helium-Neon laser was focused on the luminal surface of the artery, and a CMOS camera captured laser speckle images at 240 frames/s, as shown in FIG. 1. Time-varying laser speckle images were analyzed using cross-correlation techniques to determine the speckle decorrelation time constant, τ, which is inversely dependent on the rate of change of the speckle image. The plaques were histologically classified into the following groups: thin-cap fibroatheroma (TCFA), thick-cap fibroatheroma (TKFA), pathological intimal thickening (PIT), non-necrotic fibroatheroma (FA), intimal hyperplasia (IH), fibrous plaque, and fibrocalcific plaque (FC).¹ The average speckle decorrelation time constant, τ, was computed for each plaque group.

FIG. 2(A) shows examples of the normalized speckle decorrelation curves computed for three aortic plaques. The TCFA demonstrated rapid speckle decorrelation (τ=28 ms) as compared to TKFA (τ=265 ms) and fibrous plaque (τ=540 ms). The average exemplary speckle decorrelation time constant, τ, computed for different plaque groups under static conditions are plotted in FIG. 2(B). The results of the analysis of variance (ANOVA) and Dunnetts t-tests demonstrated highly significant differences in τ between the plaque groups (p<0.0001). TCFA's exhibited a significantly lower time constant (τ˜47 ms) as compared to other lesions due to rapid Brownian motion of endogenous particles within the compliant necrotic core (p<0.001). As a result, the exemplary LSI technique demonstrated high diagnostic sensitivity (100%) and specificity (92%) for identifying TCFA's. Fibrous and fibrocalcific lesions were also easily discriminated from lipid-containing lesions due to their significantly higher time constants.

Relationship between plaque composition and laser speckle decorrelation: The time constant, τ, shows high correlation with plaque collagen content (e.g., R=0.73; p<0.0001) when measured using polarized light microscopy of Picrosirius Red (PSR) stained sections. Further, a high correlation between z and minimum cap thickness (R=0.87; p<0.001) and a strong negative correlation (R=−0.81; p<0.0001) between τ and necrotic core area can be obtained. These exemplary results demonstrate that the exemplary LSI measurements of τ can be related to plaque collagen and lipid content.

Preliminary studies to evaluate viscoelastic modulus, G: Exemplary preliminary studies have been conducted to evaluate the bulk viscoelastic modulus, G, and its relationship with LSI measurements of τ using in homogenous collagen substrates and arterial plaques and using modeling studies. (The term, bulk viscoelastic modulus, G, can be used to define the overall modulus of the sample which integrates over the sample volume).

Homogenous collagen substrate studies: In one example, the exemplary LSI technique was performed on type I collagen gels at varying concentrations (0.2%, 0.3%, 0.4%, 0.6% and 0.8% m/v), and on cartilage disks (type II collagen) obtained from swine knees and ears. In gels, speckle decorrelation time constant, τ, showed high correlation with collagen concentration (R=0.99, p<0.002). Mechanical testing can be performed on all gel and cartilage samples using a Bohlin C-VOR rheometer (Malvern Instruments Inc., MA) to measure G(ω), (0.5<ω<10 Hz). FIG. 3 shows a graph with exemplary mechanical measurements of viscoelastic moduli of collagen gels obtained using a rheometer. In FIG. 3, τ shows a high correlation with mechanical testing (R=0.97, p<0.0001), establishing a strong relationship between LSI and viscoelasticity. Atherosclerotic plaque studies: In another example, the exemplary LSI technique was conducted by averaging τ values over 4 mm disks of arterial sites, histologically confirmed as calcific, fibrous and NCFA. Mechanical testing was performed using the Bohlin rheometer, and the modulus, G, was measured by averaging G(ω) over the linear range. The G values measured for plaque groups were distinct [2.27×105 Pa (calcific), 3.65×103 Pa (fibrous) and 2.23×103 Pa (NCFA)], and LSI measurements of τ correlated well with G (R=0.99, p<0.001). The above exemplary results indicate a close relationship between LSI measurements of τ and G(ω) measured by mechanical testing.

For all plaques, G was approximately equal to the elastic modulus, G′, suggesting that the plaques were largely elastic in the measurement regime with a small viscous modulus, G″. These values correspond with previously published reports.39 ANOVA tests showed statistically significant differences in G for the three plaque types (p<0.001).

Modeling Analysis: To evaluate the effect of depth dependent variations in plaque viscoelasticity on the bulk viscoelastic modulus, the atherosclerotic plaque can be modeled as a multilayered cylinder of thickness, L and viscoelastic modulus, G. For the purpose of this exemplary model, the assumption that G≈G′ can be made, which can be supported by ex-vivo exemplary analysis above. As an initial matter, the case of a NCFA can be considered, consisting of a fibrous cap layer of thickness L1 with modulus G1, overlying lipid pool layer of thickness L2 with modulus G2, loaded between the parallel plates of a rheometer. The twisting moment M applied by the rheometer can be determined by the distribution of shear stresses, T, integrated across the cylinder of cross-sectional area, A. The moment, M, is given by:

$\begin{matrix} {M = {{\int{{rt}{\mathbb{d}A}}} = {G\frac{\mathbb{d}\varphi}{\mathbb{d}z}I_{z}}}} & ({C1}) \end{matrix}$ where I_(z)=∫r² dA is the polar moment of inertia and φ is the angular displacement in the sample. Since the moment M=M1=M2 for each layer, by evaluating equation (C1) for each layer of thickness L1 and L2, and for the entire cylinder, L we deduce the expression:

$\begin{matrix} {G = \frac{{LG}_{1}G_{2}}{{L_{1}G_{2}} + {L_{2}G_{1}}}} & ({C2}) \end{matrix}$

Equation (C2) shows that the overall bulk viscoelastic modulus of the plaque is related to the thickness and viscoelastic modulus of each layer. This equation (C2) can be applied to evaluate the relationship between the bulk modulus G and fibrous cap thickness in a NCFA, using previously reported values of G1=496 kPa, and G2=222 kPa, for fibrous and lipid rich tissue and evaluated the influence of varying fibrous cap thickness (e.g., 0-500 μm) on G (as shown in FIG. 3). This exemplary model can be extended to include multiple layers of varying depth-dependent viscoelasticity by using the generalized equation:

$\begin{matrix} {\frac{L}{G} = {\sum\limits_{n}\frac{L_{i}}{G_{i}}}} & ({C3}) \end{matrix}$

These analyses suggest that the fibrous cap thickness greatly influences the bulk viscoelasticity of the plaque (see FIG. 3), indicating that the measurement of the bulk viscoelastic modulus can potentially provide a quantitative metric to evaluate plaque stability.

Measuring tissue heterogeneity using Exemplary LSI Systems and Techniques: In the exemplary analyses described above, bulk viscoelasticity can be evaluated over the entire speckle image by illuminating a single location. Thus, the Brownian motion was integrated over the illuminated volume and information about tissue heterogenity was lost. These exemplary analyses have provided significant evidence to show that measuring bulk viscoelasticity alone can provide an important metric related to plaque stability. However, evaluation of compositional and structural heterogeneities provides additional information about the risk of rupture. Exemplary analyses described below determine the feasibility of in evaluating both, (i) spatial, and (ii) depth-dependent heterogeneities in viscoelastic properties using laser speckle.

Laser speckle to evaluate spatial (or transverse) heterogeneity: Laser speckle images can be obtained by scanning the laser beam at small spatial increments and the spatial distribution of τ can be measured across the plaque. FIGS. 4(A)-4(D) illustrate the transverse spatial variation of τ as a function of beam location. As the beam was scanned across each lesion, τ varied significantly depending on tissue type: τ was low (20-50 ms) in the necrotic core regions 405 (see graph 400—shown in FIG. 4(A)) and higher in the calcific 415 (˜2200 ms as shown in graph 410 in FIG. 4(B)) and fibrous 425 (˜800 ms as shown graph 420 in FIG. 4(C)) regions. FIG. 4(D) illustrates a two-dimensional map 435 of the spatial distribution of τ, measured by scanning the beam at 300 μm increments across a lipid-rich plaque: a well-demarcated region 430 of low τ relative to the surrounding aortic tissue is seen. These exemplary results show that beam scanning can be utilized to evaluate spatial variation in plaque viscoelasticity to potentially detect heterogeneities such as calcific nodules and localized necrotic cores.

Laser speckle to evaluate depth heterogeneity: Due to the diffusive properties of light propagation in tissue, photons returning from deeper regions have a higher probability of remittance farther away from the illumination location. While beam scanning can provide information about spatial heterogeneities, depth-dependent heterogeneities can be measured by analyzing variation in τ as a function of radial distance, ρ, from the illumination location in each speckle image. An exemplary embodiment of the method and system according to the present disclosure can be provided to obtain depth-dependent measurements by combining spatio-temporal laser speckle analysis with diffusion theory and Monte Carlo models of light propagation. Such exemplary method and system can be used to measure fibrous cap thickness in necrotic-core fibroatheromas (NCFA's), which can also be applied to evaluate depth-dependent viscoelasticity.

For example, laser speckle patterns of 20 NCFA's were analyzed and spatio-temporal speckle fluctuations were measured by exponential fitting of the windowed normalized cross-correlation of sequential speckle patterns to obtain τ(ρ). By analyzing the spatial variation in τ(ρ) the distance, ρ′, can be obtained at which τ(ρ) dropped to 65% of its maximum value. FIG. 5(A) shows a graph of τ(ρ) plotted vs. distance ρ from source and FIG. 5(B) shows an exemplary schematic illustration of a photon propagation through a two-layer model. For example, FIG. 5(A) shows that τ(ρ) is plotted vs. distance ρ from source. At distances <ρ′, e.g., most photons traverse the fibrous cap and τ(ρ) is high. At distances >ρ′, e.g., a majority of photons traverse the necrotic core and τ(ρ) is low. A Monte Carlo look up table can be created to relate radial distance, ρ, across the speckle pattern to the maximum photon penetration depth, z_(max)(ρ), through the plaque. To measure cap thickness, the depth z_(max)(ρ′), can be evaluated at ρ=ρ′, that highly correlated with histological measurements (R=0.78, p<0.0001). Paired t-tests showed no significant difference from histological measurements (p=0.4). These exemplary findings indicate the possibility of measuring spatial and depth-dependent viscoelasticity using LSI, potentially providing insight into structural and compositional heterogeneities.

Validation of Exemplary Methods to Measure the Viscoelastic Properties of Arterial Tissue from Laser Speckle Images

Overview: Measurement of Viscoelastic Properties Using Laser Speckle

An exemplary embodiment of the method, computer-accessible medium and system to measure viscoelastic properties of atherosclerotic plaques from laser speckle images can be based on previously-established optical methods. For example, using certain dynamic light scattering techniques, a quantity termed the mean square displacement (MSD),

Δr²(t)

, can be measured which provides an assessment of scatterer motion such as Brownian motion in the tissue. The MSD can be related to the material's frequency-dependent viscoelastic modulus, G*(ω). To probe the mechanical properties of highly scattering media such as colloids and polymer gels, Diffuse Wave Spectroscopy (DWS) techniques can be utilized. With the exemplary DWS techniques, a laser beam can be provided incident on the sample and light scattered multiple times is collected using a single optical fiber in transmission or backscattering geometry.

To measure the Brownian motion dynamics of the ensemble of particles within the medium, the time-varying intensity fluctuations over a single speckle spot can be measured by averaging over several cross-correlation functions that evolve in time to obtain the function, g₂(t). The ensemble speckle cross-correlation function, g₂(t), can be used to measure the MSD and the resulting elastic, G′(ω), and viscous, G″(ω), moduli and the resulting complex modulus G*(ω). These exemplary methods have been demonstrated in the field of polymer rheology to measure the viscoelastic properties of homogenous materials such as semiflexible actin gels, polyacrylamide networks and other complex fluids. In standard DWS, since g₂(t) is measured over a single speckle spot, data acquisition time is several orders of magnitude larger than the typical time scale of fluctuations (e.g., acquisition time of several minutes) which can be impractical for materials that exhibit slower particle dynamics. Advances in DWS technology have described the use of CCD cameras to simultaneously acquire multiple speckles over the sample (“multispeckle” DWS). This exemplary technique enhances the statistical accuracy in determining the MSD by simultaneous ensemble averaging of multiple speckle spots, significantly reducing the data acquisition time to a few ms. Previous analyses in polymer rheology indicated that g₂(t) functions measured using ‘multispeckle DWS’ and standard DWS show high agreement with an error <about 2%.

With the results from the exemplary “multispeckle” DWS techniques to measure polymer viscoelasticity, it is possible to apply these exemplary techniques to evaluate the viscoelastic properties of human tissue, e.g., atherosclerotic plaques. The term “LSI” as used herein can be similar to the “multispeckle” DWS described in polymer rheology applications, but certainly not limited thereto. For plaque measurements, the evaluation of two-dimensional speckle images (e.g., in the exemplary LSI techniques and systems) instead of a single speckle spot (e.g., in standard DWS) can be advantageous as it can provide more complete information about tissue viscoelastic properties, and facilitate depth-resolved measurements potentially enabling the evaluation of important parameters such as the thickness and viscoelasticity of the fibrous cap and necrotic core. The exemplary procedures of such methods to provide and validate the measurement of plaque viscoelasticity using the exemplary embodiments of the LSI techniques and systems according to the present disclosure is shown in the block diagram of FIG. 6(A).

For example, as shown in FIG. 6(A), time varying laser speckle images can be acquired at high frames (block 610). Then, in block 620, such time-varying laser speckle images acquired at high frame rates can be analyzed using exemplary cross-correlation techniques to obtain the speckle decorrelation curve, g₂(t). The MSD of particle motions such as Brownian motion can be estimated or measured from the speckle decorrelation data (block 630). Parameters that characterize the medium scattering properties required to estimate the MSD can be evaluated from time-averaged laser speckle images (block 660) and by using diffusion theory and Monte Carlo simulations of light propagation through the sample (block 665). The viscoelastic or complex modulus, G*(ω) and the elastic, G′(ω) and viscous, G″(ω) moduli can be derived (block 640) and compared with standard mechanical testing measurements (block 650).

Exemplary LSI System and Instrumentation

An exemplary embodiment of the LSI system according to the present invention can be provided to acquire laser speckle images, as shown in FIG. 6(B). For example, light from an unpolarized Helium Neon light source 670 (e.g., 632 nm, 30 mW) can be coupled into an optical fiber arrangement 675, such as a single-mode fiber. The beam can be expanded by, e.g., 5:1, reflected off a galvanometer-mounted mirror 680, and focused to, e.g., a 50 μm diameter spot on the surface of a sample 685. The galvanometer-mounted mirror 680 can be computer-controlled by a computer 690 to facilitate scanning the illumination beam across the sample 685. It is also possible to illuminate the tissue surface of the sample 685 using a larger diameter extended beam or a collimated beam. At the collection end of the exemplary system, a collection arrangement 695, such as, e.g., a high-speed, digital CCD or CMOS camera (e.g., Mikrotron MC 1310) configured to acquire speckle patterns at high frame rates may be provided and images can be transferred to the computer 690 in real time. Time-varying cross polarized laser speckle images can be acquired from imaging sites on the tissue sample 685.

Exemplary Laser Speckle Image Analysis: Measurement of Mechanical Properties Such as Viscoelastic Moduli

Exemplary acquired time-varying laser speckle patterns can be analyzed using cross-correlation techniques to determine the speckle cross-correlation function, g₂(t). The normalized 2D cross-correlation of the first speckle image with each image in the time-varying image series can be determined using the exemplary embodiments of the present disclosure. The maximum value of normalized cross-correlation can be determined and plotted as a function of time over the acquisition duration to obtain the g₂(t) curve for each tissue sample. To measure the viscoelastic properties of the sample, the g₂(t) curve can be evaluated to obtain the MSD and the resulting elastic, viscous and complex moduli. The ensemble speckle cross-correlation function, g₂(t), can be expressed in terms of the MSD,

Δr²(t)

, of scattering particles as follows,

$\begin{matrix} {{{g_{2}(t)} - 1} = \left\lbrack {\int_{0}^{\infty}{{\mathbb{d}{{sP}(s)}}{\exp\left( {{- \left( {{s/3}I^{*}} \right)}k^{2}\left\langle {\Delta\;{r^{2}(t)}} \right\rangle} \right)}}} \right\rbrack^{2}} & ({D1}) \end{matrix}$ where P(s) is the distribution of photon trajectories traversing a path length, s, and k=2πn/λ, is the wave number of light in a medium where n is the refractive index and λ, the wavelength of light. The mean free path, I*, can characterize the scattering medium and is defined as the distance a photon travels before its direction is completely randomized. The exemplary embodiment of a method according to the present disclosure that can be used to estimate the parameters, P(s) and I*, preferable to calculate the MSD in equation (D1) is described below.

For example, it is possible to utilize the exemplary mathematical methods that have been derived and established in previous DWS analyses in homogenous polymers to measure G*(ω). In these exemplary methods, a modified algebraic form of the generalized Stokes-Einstein equation directly relates the MSD of probe particles to the frequency dependent viscoelastic modulus, G*(ω), of the material (equation. D2).

$\begin{matrix} {{{G^{*}(\omega)}} = {\frac{kT}{\pi\; a\left\langle {\Delta\;{r^{2}\left( {1/\omega} \right)}} \right\rangle{\Gamma\left\lbrack {1 + {\alpha(\omega)}} \right\rbrack}}❘_{t = {1/\omega}}}} & \left( {D\; 2} \right) \end{matrix}$ where G*(ω) is the frequency dependent complex viscoelastic modulus, a is the scatterer size, Γ is the gamma function, and

Δr²(1/ω)

is the magnitude of the MSD at t=1/ω. The value of α(ω) can be given by:

$\begin{matrix} {{\alpha(\omega)} = {\frac{{\mathbb{d}\ln}\left\langle {\Delta\;{r^{2}(t)}} \right\rangle}{{\mathbb{d}\ln}\; t}❘_{t = {1/\omega}}}} & ({D3}) \end{matrix}$

The particle size, a, is the characteristic length scale probed and can be given by,

$\begin{matrix} {a \approx \sqrt{\frac{1}{{k^{2}\left( {{z_{o}/I^{*}} + {2/3}} \right)}^{2}}}} & ({D4}) \end{matrix}$ where z_(o) is the penetration depth. From previous studies it is estimated that z_(o)≈0.6ρ in arterial tissue, where ρ is the radial distance on the 2D speckle image measured from the central illumination location.

Average particle sizes of different tissue types can also be estimated using an iterative process by G*(ω) using a mechanical rheometer, and retrospectively deducing particle size, a, values using equation (D2). This a priori estimate of particle size can then be applied to measure viscoelastic properties from laser speckle patterns for prospective measurements of tissue samples.

The elastic, G′(ω), and viscous, G″(ω), moduli can be determined using the following relations: G′(ω)=|G*(ω)|cos(πα(ω)/2) G″(ω)=|G*(ω)|sin(πα(ω)/2)  (D5)

These exemplary relations can provide a direct physical representation of how the elastic modulus and viscous modulus of the material depend on the MSD. In a purely viscous medium, α≈1 resulting in a dominant loss modulus, and in a purely elastic medium α≈0 and the elastic modulus dominates.

Exemplary Estimation of Parameters: P(s) and I*

The exemplary evaluation of the MSD techniques of probe particles from the g₂(t) function as expressed in equation (D1) can utilize the measurement of the distribution of photon trajectories, P(s), traversing a path length s, and the mean free path, I*. The parameters, P(s) and I* which characterize the optical properties of scattering medium can be derived from time-averaged speckle images using previously described methods. It is possible to determine optical properties of human tissue by combining a diffusion theory model of spatially-resolved diffuse reflectance⁵⁸ and a Monte-Carlo model of light transport in tissue. According to one exemplary embodiment of the present disclosure, it is possible to utilize these exemplary methods to obtain the parameters, P(s) and I*. The sample can be described by its optical parameters: the absorption coefficient, μ_(a), the scattering coefficient, μ_(s), and the anisotropy coefficient, g, as well as the refractive indices of air and tissue (n=1.4) First, it is possible to derive the optical properties of the sample by measuring the radially dependent remittance from the sample. Apriori estimates of tissue optical properties can also be used.

Time-varying speckle images of the fibrous plaque can be obtained using the imaging the exemplary embodiment of the system and method according to the present disclosure as described herein. Given the quantum efficiency and gain of the CCD camera, the total number of diffuse photons remitted from the plaque and detected by the CCD sensor can be measured by time-averaging speckle images acquired over a time duration of a few seconds or longer. The radially-resolved photon probability, P(ρ), for the fibrous plaque can be generated by summing the number of photons detected over different annuli of radii ρ, and then normalizing this value by the total number of photons detected over the area of the detector. Further, the theoretical radial photon probabilities determined from a single-scatterer diffusion model for the case of a semi-infinite homogeneous tissue 58 can be fitted to the measured radial photon probabilities, P(ρ), using a least-square optimization procedure, to extract the optical properties, μ_(a), μ_(s) and g, of the sample. The mean free path, I*, can be then evaluated for the scattering medium, which is given by I*=1/μ_(a)(1−μ_(s)). When the optical properties are established, they can be used as inputs to a Monte Carlo model which assumes a semi-infinite homogenous layer. 49 Photon initial conditions can include input beams perpendicular to the semi-infinite layer. Multiple runs can be performed with the same set of optical properties and photon packet trajectories can be launched. Remitted photons can be collected over a radial distance of a few mm. From the output of the Monte Carlo simulations, the maximum path length, s, traversed by each photon can be recorded and the path length distribution, P(s), of photons can be measured. The parameters, P(s) and I*, can be input into equation D1 to determine the MSD of probe Brownian motion in the sample.

Exemplary Methods

Exemplary LSI system optics: Optics for light delivery and speckle image transmission can be designed and optimized using, e.g., ZEMAX (ZEMAX Development Corporation). A variety of different lenses can be simulated and optimization can be performed to minimize aberrations through different optical window designs and to increase field of view. Following the optimized design and selection of configuration and components, optical elements can be obtained. The laser, illumination and collection optics, optical fibers, CCD camera, galvanometer-controlled mirror, linear translation device, and computer can be integrated in a portable cart. Software can control the motors, reading and storing motor encoder positions, laser speckle analysis, and displaying data in a various formats for ease of interpretation.

Exemplary Collagen Phantom Preparation for LSI

Since Type I collagen can be a predominant constituent of the extracellular matrix in atherosclerotic plaques, test phantoms can be made using commercially available collagen to evaluate the performance of LSI in measuring sample viscoelasticity. Collagen gels (Type I) can be constructed from rat tail tendon collagen dissolved in 0.02N acetic acid (8 mg/ml) (BD Biosciences, catalog no. 354249). Latex microspheres with a diameter of about 0.3 μm (10% in water) can be used as light scattering probes. To evaluate the influence of collagen concentration on the measured viscoelastic modulus, collagen gels can be constructed with at each collagen concentration of 0.7%, 0.5%, 0.3%, 0.2%, and 0.1% (mass/volume). Since collagen can dissolve only in an acidic medium, and forms gels only in a neutral medium of pH from about 7 to 8, a high pH buffer can be used to neutralize the acetic acid in the collagen solution. A high pH buffer can be used.

Exemplary Coronary Atherosclerotic Plaque Specimens

Cadaveric coronary arterial segments can be excised during autopsy, and slit longitudinally open to expose the luminal surface. The coronary segments can be immersed in phosphate buffered saline and warmed to 37° C. before imaging.

Exemplary Laser Speckle Imaging of Samples

Time-varying laser speckle images of the coronary arterial specimens and collagen gel phantoms can be obtained over a measurement duration of about 1s, according to one exemplary embodiment of the present disclosure. For example, each sample can be stabilized on a cork-board, clamped onto an L-brackets mounted on a linear motorized stages. The L-brackets can be immersed in a PBS bath maintained at about 37° C. such that the luminal surface of the artery (or surface of the collagen gel) is exposed just above the level of PBS. In the exemplary arterial specimens, time-varying laser speckle images can be obtained at randomly selected discrete lesion sites along the segment. Each imaging site can be marked with two India ink spots marking the diameter of the speckle pattern over the lesion, to facilitate accurate registration with mechanical testing measurements and histopathology. Circular sections can be cut across the artery at each marked imaging site using a 2 mm circular punch biopsy too and stored in, e.g., PBS. In the collagen gel phantoms laser speckle images can be obtained three randomly selected sites for each gel to evaluate heterogeneity. In all samples, the frequency-dependent viscoelastic modulus, G*(ω), can be computed at each spatial location from the time-varying laser speckle images using techniques described above. Following LSI, the samples can be prepared for standard mechanical testing procedures.

Exemplary Mechanical Testing

Mechanical testing to measure the viscoelastic properties of the coronary arterial specimens and collagen gel phantoms can be performed using a Bohlin C-VOR computer-controlled mechanical rheometer (e.g., Malvern Instruments, Southborough, Mass.). An exemplary embodiment of the system according to the present disclosure can include two parallel plates that can hold the sample affixed to the bottom plate to prevent slipping. A shear stress can be delivered to the sample by the motor via an oscillatory torque applied to the top plate. The resultant strain in the sample can be measured by an angular position sensor incorporated in the exemplary system and automated Bohlin system software can calculate G*(ω), G′(ω) and G″(ω). The mechanical testing can be conducted at, e.g., about 37° C. In the first stage of mechanical testing, a gradually increasing stress can be applied and the strain response can be recorded.

The resultant viscoelastic modulus, G*, can be plotted as a function of measured strain to determine the range of linear strain response over which G* is independent of strain to provide an estimate of the mechanical strength of each sample. The threshold strain, γ_(max), can be determined above which the sample's intermolecular forces are overcome by the stress and the sample viscoelastic modulus falls. In the second stage of mechanical testing, an oscillatory strain can be induced in the sample swept through a frequency range, 1<ω≦100 Hz. The maximum strain can be maintained at γ_(max). The lower limit of the frequency range can be determined by the imaging time (1 s) over which the exemplary LSI techniques is performed and the upper limit, ω=100 Hz, can be limited by, e.g., the maximum frequency limit of the Bohlin rheometer system. The frequency dependent viscoelastic, G*(ω), elastic, G′(ω) and viscous, G″(ω), modulii can be recorded for each sample.

Exemplary LSI measurements of viscoelastic moduli performed in the coronary specimens and collagen gel phantoms can be compared with mechanical testing and Histological measurements of plaque collagen content. Exemplary results are shown below:

Single location measurements: Using the exemplary methods described herein, in one example, the techniques above were applied to measure viscoelastic moduli from laser speckle patterns of animal tissue specimens such as cartilage, muscle and fat. In this example, the overall “macro” viscoelastic modulus of bulk tissue within the illuminated volume can be measured from the MSD data determined over the entire speckle pattern obtained by focusing the beam to a 50 μm spot. The exemplary results are shown in FIG. 7A which illustrates the bulk viscoelastic moduli measurements plotted as a function of frequency measured from laser speckle patterns of cartilage 700, skeletal muscle 705, and adipose fat at temperatures of 4° C. 710 and 40° C. 715. The exemplary results indicate that cartilage 700 has highest modulus values compared to skeletal muscle 705 and adipose fat 710, 715. Additionally, temperature influences sample viscoelasticity evidenced here by a lower modulus measured for adipose fat at 40° C. 715 compared to that at 4° C. 710.

In another example, the exemplary techniques described herein above were applied to human atherosclerotic plaques. FIG. 7B shows the exemplary LSI measurements of plaque viscoelasticity obtained from human atherosclerotic plaques. The results demonstrate that higher moduli measurements were measured for the calcific plaques 720 and fibrous plaques 725 compared to the lipid-rich plaque 730. At higher frequency regions of the G(ω) plot, the viscoelastic modulus of fibrous plaque 725 was significantly higher than lipid rich tissue 730 compared to the lower frequency regions.

Viscoelasticity mapping: While laser speckle patterns obtained by beam focusing (as described above) can provide information about tissue viscoelasticity over the illuminated volume, scanning a collimated or focused beam over a sample can facilitate the evaluation of spatial heterogeneities in viscoelastic moduli. For example, FIGS. 8(A)-8(D) show an example of two-dimensional maps of the frequency dependent modulus, G(ω), measured by scanning a 5 mm collimated beam over a 5 cm region of a human cadaveric artery. In this case, G(ω) values were computed from MSD data measured within overlapping windowed regions of 100×100 μm over the artery. Two-dimensional maps of G(ω) can be obtained by performing an interpolation over the region of interest. In FIGS. 8(B)-8(D), G(ω) maps computed at different frequencies are plotted, respectively, and compared with a gross pathology photograph (FIG. 8(A)) of the artery, in which calcific regions 800, fibrous regions 805 and lipid-rich regions 810 are demarcated. The India ink spot 815 is also visible in the maps, and can used for accurate registration of the G(ω) maps with the gross pathology image. As seen in G(ω) plots, at higher frequencies the calcific tissue types 820, fibrous tissue types 825 and lipid-rich tissue types 830 are distinguished by significant differences in their viscoelastic moduli. At lower frequencies (shown in FIGS. 8(C) and (D)), G(ω) differences between fibrous and lipid-rich tissue types are not highly significant. These exemplary results demonstrate the ability to measure heterogenous moduli by beam scanning simultaneously over a large range of frequencies using a non-contact optical approach. Another exemplary method to accomplish two-dimensional mapping of tissue viscoelastic moduli can be performed by illumination using an extended beam and the resulting speckle patterns can be analyzed by employing windowed over a varying range of scales (microscopic, mesoscopic and macroscopic), of g₂(t) over a single speckle spot or multiple speckle spots over the illuminated tissue. Thus, it is possible to evaluate tissue viscoelastic moduli over a varying range of scales (microscopic, mesoscopic and macroscopic).

Exemplary Method to Monitor Changes in Tissue Mechanical Properties During Disease Progression: Changes in Arterial Viscoelastic Properties Using LSI During Plaque Progression in a Mouse Model of Atherosclerosis

For example, mechanical strength of the plaque, determined by the viscoelastic modulus, G*, can be modified and compromised during plaque progression. By monitoring G* during different stages of atherogenesis, quantitative biomechanical markers can be used to evaluate the risk of rupture.

Exemplary Monitoring Changes in Arterial Viscoelasticity During Plaque Progression

It is possible to use the exemplary methods according to the present disclosure as described herein above to monitor arterial viscoelastic moduli during different stages of atherosclerosis progression in a murine model. For example, it is possible to evaluate the influence of multiple factors on the arterial viscoelasticity specifically: stage of atherogenesis (imaging time point), plaque type and blood cholesterol. Mice on a high fat diet can be investigated at four imaging time points. LSI of murine aortic, brachiocephalic, carotid arteries and the iliac bifurcation can be conducted. Time-varying laser speckle images can be analyzed to measure arterial viscoelastic moduli. Arterial viscoelasticity can be serially monitored at each imaging time point and compared with Histopathological findings at sacrifice.

Further Exemplary Methods

Below, exemplary embodiments of the methods and systems according to the present disclosure to evaluate and monitor arterial viscoelastic properties during atherosclerosis progression in atherosclerotic mice can be utilized.

Exemplary Mouse Model of Atherosclerosis

Exemplary use of atherosclerotic mouse model: An atherosclerotic mouse model using Apolipoprotein E knockout (ApoE −/−) mice (background strain—C57BL/6) can be used to review this exemplary embodiment. This exemplary model can be based on previous analyses which indicated that advanced necrotic core plaques resulting in plaque rupture occurred in apoE-knockout mice after 8 weeks of fat feeding. The exemplary LSI analyses can be implemented to (i) evaluate the use of apo E−/− mice to evaluate plaque progression, and (ii) to test the feasibility of measuring viscoelasticity of mouse arteries using laser speckle techniques.

The apolipoprotein E knockout (ApoE −/−) murine model has been shown to be a reliable and reproducible model for atherosclerosis, and its lesion characteristics are similar to those associated with plaque instability in humans. The feasibility of measuring arterial viscoelasticity of aortic plaques can be assessed in apo E−/− mouse arteries. In one study, segments of the abdominal aorta were obtained from a fat fed apo E−/− mouse at 14 weeks. For example, an exemplary LSI procedure was conducted by scanning a focused (20 μm) beam (632 nm) and measuring τ, at 300 μm increments along the length of the aorta. FIG. 9 shows the spatial distribution of τ 900 co-registered with the corresponding gross pathology photograph of the mouse aorta 905, and measured by beam scanning which shows evidence of fibrous plaque with varying mechanical properties. The τ value varied significantly and was higher in region corresponding the location of the plaque (τ=462 ms) suggesting the presence of a fibrous plaque. Lower τ values adjacent to the fibrous plaque may be attributed to hyperlipidemia in the apo E−/− mouse. This exemplary data indicates that by beam focusing in conjunction with scanning, the exemplary LSI technique and system according to the present disclosure can detect plaques in mouse arteries

Multiple mice can be used; for example, 48 C57BL/6 ApoE −/− and 12 regular C57BL/6 mice (control) can be studied. Starting at about 6 weeks of age, the 48 ApoE −/− mice can be placed on a high fat diet (e.g., 0.2% cholesterol, 21% fat, Harlan Tekland #88137) and 12 control mice continued on a regular chow diet (0% cholesterol, 5.7% fat, Harlan Tekland #2018). For example, the first imaging time point can be at 6 weeks after initiation of the high fat diet. At each time point, 12 ApoE −/− and 3 control mice can be randomly selected and sacrificed. The mouse vasculature can be prepared for imaging and LSI measurements along with corresponding Histopathology can be performed on each animal (as described below). Subsequently, the second, third and forth imaging time point can be, e.g., at 12 weeks, 18 weeks and 24 weeks after initiation of the high fat diet. LSI and Histopathological measurements can proceed in the manner described for the first imaging time point. At each imaging time point, blood samples can be drawn from both ApoE −/− and control mice, and total cholesterol can be determined enzymatically. The exemplary sites of lesion prediliction in the apoE −/− mouse aorta are shown in FIG. 9.

Exemplary Laser Speckle Imaging of Mouse Vasculature

Development of atherosclerotic lesions in the vasculature of mice can occur at reproducible sites which are predominantly dictated by heinodynamic forces experienced by the endothelium. Thus, it is possible to select arterial segments in the mouse vasculature to conduct LSI to coincide with arteries exhibiting plaque predilection. The aorta (including the ascending, thoracic and abdominal aorta), brachiocephalic trunk, right and left common carotids and the iliac bifurcation can be imaged using for LSI. Similarly, the brachiocephalic trunk, and the left and right common carotid arteries can be imaged in 1 mm increments advancing from the aortic arch towards the carotid bifurcation. Time-varying laser speckle images can be obtained at high frame rates at each imaging site over a measurement time duration determined using the exemplary embodiments described herein above.

Exemplary Histological Processing and Analysis

Following the exemplary imaging, the arterial segments can be fixed in about 10% formalin, embedded and sectioned using standard Histology techniques. Cross-sectional sections (thickness=4 μm) can be cut over the length of each aortic, brachiocephalic and common carotid arteries. The sections can be stained with H & E and Trichrome stains, and interpreted by a pathologist blinded to the LSI data. Atherosclerotic lesions and the natural history of their progression in the apoE-knockout mouse bear a resemblance to atherogenesis in humans. Fatty streaks are present in early stages and as lesions progress multilayered appearances occur showing presence of smooth muscle cells. Advanced lesions indicated fibrous cap appearance, necrotic core, cholesterol clefts and calcifications. Spontaneous plaque rupture has been shown in fat fed mice with the fibrous cap significantly thinner in ruptured lesions than intact lesions. Due to the similarities with human atherosclerosis, it is possible to characterize atherosclerotic lesions in apoE-knockout mice based on the classification scheme proposed by Virmani et al. Atherosclerotic lesions can be classified into the following six groups: intimal xanthoma (or fatty streak), intimal thickening (IT), necrotic core fibroatheroma (NCFA), ruptured plaque, fibrous plaque and calcific plaque.

Exemplary Statistical Analysis

Exemplary time-varying laser speckle images obtained from the mouse vasculature can be evaluated using the exemplary techniques described herein above. The frequency dependent viscoelastic modulus, G*(ω), can be measured from the mean square displacement of plaque particles which will determined from the speckle cross correlation curve, g₂(t). The value of the viscoelastic modulus, G* at the optimal frequency, ω, (as described herein above) can be recorded from the G*(ω) data. Based on histological diagnoses, the G* value associated with each lesion can be assigned to one of six classified plaque groups for each of the four imaging time points. For each plaque type, the G* data can be expressed as G*±s_(G)*, where G* is the average viscoelastic modulus computed for each plaque group at each imaging time point and s_(G)* is the standard deviation.

Multiple factors can influence the viscoelastic modulus, G*. For example, the influence of following factors on G* can be evaluated: number of weeks on high fat diet, plaque type, animal within each plaque group, and blood cholesterol at each time point. The differences between G* measurements influenced by these factors can be evaluated using three-way analysis of co-variance tests. The three factors included in the analysis can be: imaging time point, plaque type, and animal within each plaque group. To determine whether blood cholesterol is a determining factor that influences the value of G*, the covariate in these tests can be the measured blood cholesterol level at each imaging time point. Statistical significance to elucidate differences in G* measurements for the tests can be defined by a p-value <0.05. Fibrous cap thickness in the NCFA group can be determined from digitized Trichrome-stained histology sections. The relationships between G* and fibrous cap thickness in the NCFA set can be investigated using linear regression.

The foregoing merely illustrates the principles of the invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. Indeed, the arrangements, systems and methods according to the exemplary embodiments of the present disclosure can be used with and/or implement any OCT system, OFDI system, SD-OCT system or other imaging systems, and for example with those described in International Patent Application PCT/US2004/029148, filed Sep. 8, 2004 which published as International Patent Publication No. WO 2005/047813 on May 26, 2005, U.S. patent application Ser. No. 11/266,779, filed Nov. 2, 2005 which published as U.S. Patent Publication No. 2006/0093276 on May 4, 2006, and U.S. patent application Ser. No. 10/501,276, filed Jul. 9, 2004 which published as U.S. Patent Publication No. 2005/0018201 on Jan. 27, 2005, and U.S. Patent Publication No. 2002/0122246, published on May 9, 2002, the disclosures of which are incorporated by reference herein in their entireties. It will thus be appreciated that those skilled in the art will be able to devise numerous systems, arrangements and methods which, although not explicitly shown or described herein, embody the principles of the invention and are thus within the spirit and scope of the present disclosure. In addition, to the extent that the prior art knowledge has not been explicitly incorporated by reference herein above, it is explicitly being incorporated herein in its entirety. All publications referenced herein above are incorporated herein by reference in their entireties. 

What is claimed is:
 1. An apparatus, comprising: at least one first source arrangement which is configured to apply at least one first coherent radiation to at least one portion of an anatomical structure; and at least one second arrangement includes a detector which is configured to receive at least one second coherent radiation from the at least one portion, the first and second coherent radiations being (i) associated with one another and (ii) time varying, wherein the at least one second arrangement is further includes a computer which is configured to determine at least one material property of the anatomical structure which is an viscoelastic modulus, an elastic modulus or a viscous modulus of the anatomical structure based on the at least one second coherent radiation, and wherein the determination by the computer is performed without at least one of: i. any portion of the apparatus causing an induction of at least one mechanical deformation on or in the anatomical structure, or ii. a requirement for the mechanical deformation on or in the anatomical structure.
 2. The apparatus according to claim 1, wherein at least one of the at least one first coherent radiation or the at least one second coherent radiation is an electro-magnetic radiation.
 3. The apparatus according to claim 1, wherein the at least one first arrangement is configured to scan the anatomical structure at multiple locations.
 4. The apparatus according to claim 3, wherein the at least one first arrangement scans the anatomical structure at the multiple locations simultaneously.
 5. The apparatus according to claim 3, wherein the at least one first arrangement scans the anatomical structure at the multiple locations sequentially.
 6. The apparatus according to claim 3, wherein the detector is further configured to detect a scan of the anatomical structure at the multiple locations simultaneously.
 7. The apparatus according to claim 3, wherein the detector is further configured to detect a scan of the anatomical structure at the multiple locations sequentially.
 8. The apparatus according to claim 1, wherein the computer determines the viscoelastic modulus or the viscosity viscous modulus that is spatially-varying or depth-varying.
 9. The apparatus according to claim 1, wherein the at least one second arrangement is configured to determine the at least one material property that is a macroscopic property of the anatomical structure.
 10. The apparatus according to claim 1, wherein the at least one second arrangement is configured to determine the at least one material property that is a microscopic property of the anatomical structure.
 11. The apparatus according to claim 1, wherein the at least one second arrangement is configured to determine the at least one material property that is a mesoscopic property of the anatomical structure.
 12. The apparatus according to claim 1, wherein the at least one second arrangement is configured to determine the at least one material property as a function of frequencies of motion of scatterers within the anatomical structure.
 13. The apparatus according to claim 12, wherein the at least one second arrangement is configured to utilize the motion of the scatterers within the anatomical structure that is a Brownian motion.
 14. The apparatus according to claim 1, wherein the at least one first source arrangement is configured to apply the at least one first coherent radiation which is a light that is scattered 2 or more times within at least one portion.
 15. The apparatus according to claim 1, wherein the at least one first source arrangement which is configured to apply the at least one first coherent radiation which is a light that is scattered one time within at least one portion.
 16. The apparatus according to claim 1, wherein the at least one first source arrangement which is configured to apply the at least one first coherent radiation that is coherent speckle.
 17. The apparatus according to claim 1, wherein the at least one first source arrangement has a configuration that applies the at least one first coherent radiation to at least one portion that is in-vivo.
 18. The apparatus according to claim 1, wherein at least one of the at least one first source arrangement is configured to apply the at least one first coherent radiation that is an acoustic radiation, or (ii) the at least one first source arrangement is configured, by applying the at least one first coherent radiation to the at least one portion, to cause an emission of the at least one second coherent radiation that is an acoustic radiation.
 19. The apparatus according to claim 1, wherein the determination by the at least one second arrangement is performed by measuring a displacement of intrinsic light scattering particles in the anatomical structure.
 20. The apparatus according to claim 19, wherein the determination is further performed based on a coherent speckle of the at least one first coherent radiation.
 21. The apparatus according to claim 19, wherein the determination of the viscoelastic modulus is based on the measurement of the displacement.
 22. The apparatus according to claim 1, wherein the determination of the at least one property is based on from a time averaged speckle of the at least one first coherent radiation.
 23. The apparatus according to claim 1, wherein the determination by the at least one second arrangement is performed by considering time-varying intensity changes of the second coherent radiation that are caused by intrinsic displacements of particles in the anatomical structure.
 24. A method, comprising: applying at least one first coherent radiation to at least one portion of an anatomical structure; receiving at least one second coherent radiation from the at least one portion, the first and second coherent radiations being (i) associated with one another and (ii) time varying; and determining at least one material property of the anatomical structure which a viscoelastic modulus, an elastic modulus or a viscous modulus of the anatomical structure based on the at least one second coherent radiation, wherein the determination is performed without at least one of: i. any portion of an apparatus performing the method causing an induction of at least one mechanical deformation on or in the anatomical structure, or ii. a requirement of a mechanical deformation on or in the anatomical structure.
 25. An apparatus, comprising: at least one first source arrangement which is configured to provide at least one first coherent radiation; and at least one second arrangement includes a detector which is configured to receive at least one second coherent radiation from at least one portion of a tissue structure, wherein the second coherent radiation is time varying, wherein the at least one second arrangement further includes a computer which is configured to: i. measure a speckle decorrelation of the at least one portion of the tissue structure using data provided by the second coherent radiation, and ii. determine a viscoelastic modulus, an elastic modulus or a viscous modulus of the tissue structure using data from the at least one measured portion, and wherein the determination by the computer is performed without at least one of: i. any portion of the apparatus causing an induction of at least one mechanical deformation on or in the tissue structure, or ii. a requirement for a mechanical deformation on or in the tissue structure.
 26. The apparatus according to claim 25, wherein the determination by the at least one second arrangement is performed by measuring a displacement of intrinsic light scattering particles in the anatomical structure.
 27. The apparatus according to claim 26, wherein the determination is further performed based on a coherent speckle of the at least one first coherent radiation.
 28. The apparatus according to claim 26, wherein the determination of the viscoelastic modulus is based on the measurement of the displacement.
 29. The apparatus according to claim 25, wherein the determination is based from a time averaged speckle of the at least one first coherent radiation.
 30. The apparatus according to claim 25, wherein the determination by the at least one second arrangement is performed by considering time-varying intensity changes of the second coherent radiation that are caused by intrinsic displacements of particles in the anatomical structure.
 31. The apparatus according to claim 25, further comprising at least one scanner arrangement which is configured to scan the at least one portion of the tissue structure along a surface thereof with the at least one first coherent radiation. 